Absolute block signaling system for railroads



Feb. 26, 1963 e. o. FERM ETAL ABSOLUTE BLOCK SIGNALING SYSTEM FOR RAILROADS Filed Dec. 31, 1957 2 Sheets-Sheet 1 554m mmv INVENTORS G. O. FERM AND G. W..DAV|SON 7 THEIR ATTORNEY I I I Feb. 26, 1963 G. o. FERM ETAL ABSOLUTE BLOCK SIGNALING SYSTEM FOR RAILROADS 2 Sheets-Sheet 2 Filed D60- 31, 1957 Auk/E mm;

6. O. FERM AND. G. W. DAVISON THEIR ATTORNEY 3,079,495 ABSOLUTE B52306; SEGNAHNG ilYdTEM FSR RAiLRGAES Glenn 0. Ferm and Gordon W. Davison, Rochester, N.Y.,

ass'gnors to Gene al Railway Signal Company, Rochester, NH.

Filed Dec. 31, 1957, Ser. No. 766,432 7 Claims. (1. ere-es This invention relates to absolute block signaling systems for railroads, and it more particularly pertains to such systems wherein signals governing entrance to a block are controlled by coded track circuits through that block.

In an absolute block signaling system, signals are provided for governing entrance in both directions to a block in a stretch of single track extending between two points, such as between two interlockings. Only one train is permitted in this block at one time, thus there is no need for intermediate signals. The block may have only a single track circuit, or there may be repeater track circuits in the block if the block is too long for a single track circuit. The opposing signals are necessarily interlocked to allow only one opposing train to enter the block in case two opposing trains aproach the block simultaneously.

Generally speaking, and without attempting to define the scope of the present invention, the system according to the present invention provides for the control of the signals governing entrance to a block by coded track circuits that are normally shut down, but are rendered active upon the approach to the block of a train from either direction.

The coded track circuits are maintained active only until a train enters the block and leaves the approach track section. Thus the track circuit coding apparatus is only active for a small percentage of the time. In addition to the system having utility in saving wear on the code communication apparatus and saving energy when no train is present in the block, there is a considerable additional saving of track circuit energy because the trackway of the block has its track circuits deenergized substantially all of the time the block is occupied by a train. Such a system is of utility particularly on railroads where primary batteries are required for the energization of the track circuits.

When a train enters the approach section in approach of a signal, a negative pulse is transmitted through the track rails to the other end of the block. This checks occupancy of the block and calls for a positive reply pulse to be transmitted for clearing the signal. The polarity of the pulse received at either end of a blocs thus determines whether the pulse has been transmitted for clearing the signal at the associated end, or whether it has been initiated at the other end of the block in accordance with the presence of an opposing train. In case two opposing trains approach at the same time, only one of the signals is cleared in accordance with a selection made by the picking up of a trafiic direction relay at one end of the block. Tracie circuit coding through the block is shut down when a train accepts an entering signal.

A directional stick relay is provided at each end of the block to prevent initiation of coding in the block when an approach track section is occupied by a train leaving the block.

An object of the present invention is to provide for the control of opposing signals governing entrance to a block by normally shut down coded track circuits in an automatic absolute block signaling system.

Another object of the present invention is to shut down coding through the trackway of a block in advance of a train when the train enters the block.

Another object of the present invention is to use Zifi'ihgldd Patented Feb. 25, 1%53 2 polarized pulses transmitted in both directions through the block as a means for initiating coding and clearing an entering signal when a train enters an approach track section for that signal.

Another object of the present invention is to automatically start track circuit coding in a block when a train enters a section of track in approach of the block, but not when a train occupies that approach track section when leaving the block.

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference is made to the accompanying drawings in which corresponding letter reference characters are used to identify relays having similar features and functions and in which FIGS. A and 15 when placed end to end illustrate schcmatical ly a circuit organization provided according to the present invention for the control of signals governing entrance to a block for opposite directions of trafiic.

Conventional schematic illustrations have been employed in the drawings to more particularly facilitate the disclosure of the system as to its general organization and mode of operation, rather than to show the specific construction and arrangement of parts that may be provided by those skilled in the art in their practice of the invention. The symbols and have been used to designate connection to the respective positive and negative terminals of suitable batteries or other sources of direct current.

With reference to FIGS. 1A and 1B, the track layout illustrated comprises a stretch of single track lltl which may be considered as connecting two interlocking plants as represented by the track switches 1T3 and ZTS respectively.

A block is illustrated as comprising a single track section T, it being indicated that repeater track sections may be added within the block as required it" the block is too long for a single track section.

Signals E and W are provided for governing entrance to the block for eastbound and westbound trafiic respectively. These signals are illustrated as being of the Searchlight type but it is to be understood that other types of signals may be used in accordance with the requirements or practice.

Adjoining approach track sections TWT and ZWT are provided for the approach of signal E, and similarly the adjoining track sections liET and ZET are provided in appr ach of signal W. Track circuits for these track sections are used in governing the initiation of a normally inactive coded track circuit that is provided for the block extending between the opposing signals E and W. Track relays TR are provided for the respective approach track sections, and they are assumed to be controlled by conventional track circuits, except that the relays lWTR and lETR require the energization of the associated relays ZWTR and ZETR respectively before they can be restored to their normally energized positions following passage of a train.

The track section T has a code following track relay associated therewith at each end of the block, track relay ETR being provided at the left-hand end of the track section T, and the track relay WTR being provided at the right-hand end of the track section T. Each of these track relays is of the code following double armature type wherein one armature is responsive only to positive pulses and the other armature is responsive only to negative pulses. Parts of the relay associated with positive and negative polarity of energization are identified by the letters P and N respectively. The structure of a relay of this general character is disclosed, for example, in the prior ac ea e U5. patent application of G, E. Marsh, Serial No. 326,- 564, filed December 17, 1952, which has resulted in Patent No. 2,832,866, granted April 29, 1958.

Codetransmitter relays ECP and WCB are provided at the left-hand and right-hand ends respectively of the track section T for the transmission of code pulses, and these relays are of the double armature type similar to, the relays that have been described as being usedv for receiving track code pulses transmitted through, the track rails of the. block.

. A pulseflength; relay, PL is provided at; each end oi the block for measuring the length of each pulse transmitted at that end, and -also for measuring, the ofi periods b etwe'en pulses, under certain conditions;

A time delay relay TD is provided at each end; of the block for selectively, connecting the track rails either to the track relay TR at that end, of the block, or to contacts, of the associated transmitting relay CP.

Code receiving relays H and D. are provided at the ends. of the block respectively for the purpose of receiving the track code pulses. Each of these relays is made sufficiently slow acting by a condenser shunt circuit to be maintained steadily picked up in response to reception; of a series of pulses. The, relays H respond to only negative pulses, and the relays'D respond to only positivev pulses. The relay D is condenser energized, and will remain picked up only if the positive armature of the associated track relay is pulsed.

Relays F5 are provided at the respective ends of the block for use in clearing the signals.

East and west directional. stick relays ES and WS are provided for preventing initiation of coding in the block when a train occupies an approach track section when leaving the block.

Time locking relays EAS and WAS, and. associated timer relays TER, are provided to protect against changing the direction of traific without the proper intervening time interval. i

Having thus considered in general the apparatus employed for one embodiment of the present invention, a more detailed consideration of the circuit organization will hereinafter be considered when consideringthe mode of operation of the system under typical operatingcondi'tions. i

' Operation The condition of the apparatus as illustrated in the drawings is the condition which exists normally with no trains present. Under these conditions, the approach track relays IlWTR and ZWTR at the left-hand end of the block are normally energized, and similarly the ap proach relays IETR and 2E TR at the right-hand end of the block are normally energized. r

The track rails at the left-hand end of the track secfir T a ne e t h W d n at ack lay. ETR through front contacts 11 and 12 of relay ETD, andsimilarly, at the right-hand end of the track section T-, the track relay WTR is connected to thfi, track rails T through front contacts 13 and 14 of relay WTD. Thus the coded track circuit for the block is normally deenergized in that both ends of the block are connected to track relays and there is no source of energy Connected to the track rails. The relay ETD which connects the track relay ETR to the track rails at the left-hand end of the block is normally energized through back contacts 15 and 16 of the positive and negative armatures respectively of the code transmitter relay ECP, which is inactive at this time. Similarly the relay WTD at the right-hand end of the block is maintained steadily energized under normal conditions through back contacts .17 and 18 of the positive and negative armatures of the codetransmitter relay WCP, which is inactive at this time.

'The time locking relays EASand WAS are normally energized'in 'acco'rdancewith the sualpracticein time locking, the relay EAS, for example, being energized in a circuit extending from including contacts 1 9 and 20 of the mechanisms of signal E, fron t conta ct 6305: relay EAS, and Winding of relay EAS, to

The general mode of operation upon the entrance of an eastbound train into the approach track section IWT isthat a negative pulse is transmitted through the block andis effective to pick up the relay WI-I at the opposite end ofthe block. With the relay WH picked up atthe righthand end of the" block, a' positive pulse is transmitted through the block for the clearing of the signal E. The first pulse received by the signal E, however, is not efiective to clear that signal'bec'ause energization of the decoding relay ED and traflic relay EFS is required for clearing the signal E. The relay ED is picked up upon.

the termination of thefirst pulse transmitted through the block, but a second pulse is required to be transmitted through the block in order to pick up the relay EFS. Coding continues by transmitting pulses in opposite directions alternately through the block, the pulses that are transmitted from the entering end being of mega: tive polarity, and the pulses transmitted from the exit end being of positive polarity. This transmission of code pulses continues in the block until the eastbound train accepts thev signalE and enters the block, and the mode of operation under these conditions will be hereinafter considered. i i l To consider more specifically the circuits providing the above described general mode of operation, the dropping away of the approach relay JgWTR causes the picking up of the negative armature N of the code transmitter relay ECP by the energization of the a Winding of this relay in a circuit extending from including back contacts 2; and 22 of the negative and positive armatures respec-- tively of the track relay ETR, back contact 23 of relay' WS, back contact 24 of relay 1WTR,'back cohtact 25 of relay EFL, back contact 26 of relay EH and winding 12 of relay ECP to The picking up of the negative: armature N of relay ECP opens back contact 16in thenormally energized circuit for relay ETD so as to causethat relay to be dropped away to disconnect the track relay ETR from the track rails by the opening of front contacts 11 and 12' respectively. The closure of back contacts 1 1 and 1'2 connects the track rails of the track section T to the negative and positive terminals respectively of the track battery 27 through front contacts 28 and 29 of relay ECP. i I i The reception of the negative pulse at the right-hand end of the track section T causes the picking up Of the negative armature of the track relay wTRpand thus pro- Y o e Pi kin u f elay WH by e energization of an obvious circuit closed through front contact 30 of relay Relay WH is made slow to drop away be: cause of its winding being shunted by resistor 31 oondenser 32v connected in series. Thus the relay WH is made sufficiently slow to drop away to be maintained P p dur n the 2 p io s be w en gat pulses.

In. espon e to th p k na up of relay W and after te at on o t ne a ve pu e, th code ra smii q relay WC-l? at the right-hand end of the track section T has its upper winding energized so as to actuate its positive armature R. 'The upper winding of relay WCPis energized upon the termination of the negative pulse by a c rcu ext nd n f o c ud g b ck contacts. 33 and 34 of the armatures N and P of track relay WTR; front contact35, of relay WH, back contact 56 of relay WPL, front contact 37 of relay WH, front contact 38 of relay WAS, back'conta-ct 39 of relay WTER, back contact .49. of relay WFS and upper winding P of relay WCP, to 1. The picking up of'this relay causes the dropping away of relay WTD,-and thus the track battery 41 is con: nected' to the track rail-s through front'co'n'tacts'42 and '43 of relay WCP so as to energize the track section T T of the positive pulse by the track relay ETR is effective through front contacts 11 and 12 of relay ETD, because this relay has become picked up by this time as a result of the termination of the negative pulse. Relay ETR, in picking up its positive armature, charges the condenser 44 through front contacts 45 and 46, but the relay ED does not become picked up until the track relay ETR becomes dropped away at the end of the pulse.

The timing or the length of the negative pulse has been effected by the picking up of the pulse length relay EPL. This relay is energized upon the picking up of relay EC? to initiate the puse, but it is made sufficiently slow in picking up so that it does not actually become picked up until time for the pulse to be terminated. in picking up, the relay EPL terminates each negative pulse by the opening of back contact 25 in the circuit for relay ECP. The circuit by which the relay ElL is energized under these conditions extends from including back contacts 21 and 22 of relay ETR, back contact 23 of relay W5, back contact 24- of relay lWTR, winding of relay EFL, front contact 16 of relay ECP and back contact 15 of relay ECP, to

Similarly at the fight-hand end of the block, the positive pulse is terminated by the picking up of the pulse length relay WPL. This relay is also slow to pick up and is energized upon the energization of the code transmitter relay WCP. The circuit by which the relay WPL is energized when a positive pulse is transmitted extends from including winding of relay WPL, and front contact 17 of relay WCP, to This relay when picked up causes the dropping away oi relay WCP to terminate the positive pulse by the opening of back contact 36 in the circuit for relay WCP.

The dropping away of relay WC? to terminate the positive pulse opens the pick up circuit that has been described for relay WPL at front contact 17, but the relay WPL is made slow to drop away because of its winding being shunted through its front contact 47, condenser 48 and resistor 49 connected in series. The time constant of this circuit is adjustable by an adjustable resistor 5% having relatively high resistance that is shunted across the condenser 48 and resistor 49 combination. The time constant of this circuit determines the rate of code transmission, and it is preferably adjusted so that the transmitter at the right-hand end of the block T will transmit at a rate of approxhnately 33 pulses per minute. Thus the pulse length relay WPL times the oil periods, and during these on periods the relay WTD is in its picked up position, thus connecting the track relay WTR to the track rails so as to receive a pulse transmitted from the opposite end of the block.

The relay EPL at the left-hand end of the block serves a similar purpose in establishing the rate of coding except that the condenser 51 and the resistor 52 are shunted across the winding of relay ElL during an ofi period in code transmission only until the next pulse is received through the track relays from the opposite end of the track circuit. This is because the shunt includes back contacts 21 and 22 of the track relay ETR. This organization is such that where dependent coding is eifective, as under the conditions being described, the code transmitter at the right-hand end of the block is considered as a driver in that it determines the rate of "transmission, while the code transmitter at the left-hand end of the block is considered as a follower in that it follows the rate determined at the right-hand end of the block, even though the shunt circuit for the relay EPL may have a time constant for a slower rate of transmission such, for example, as a rate of 29 pulses per minute.

it will therefore be apparent that a negative pulse is trans .itted from the left-hand end of the block each time the relay EPL becomes dropped away, and similarly a positive pulse is transmitted from the right-hand end of the block each time that the relay WFL becomes dropped away. The reception of the negative pulse at the right- 53 hand end of the block maintains the relay WH in its picked up position, this being a necessary qualification to maintain the code transmitter relay WCP active for the transmission of positive pulses.

Upon termination of the first positive pulse, the relay ED at the left-hand end of the block becomes picked up by the discharge of the condenser 4 This condenser 44 is of higher capacity than condenser 53 which is also shunted across the winding of relay ED through resistor 54. Thus the condenser 53 becomes charged through the resistor 54, so that it maintains energization on the relay ED during the time when code pulses are being re ceived, and when the condenser 44 is being charged through front contacts 45 and 4-5 of the track relay ET When the second positive pulse is received at the lefthand end of the block, the relay EFS becomes picked up by the energization of a circuit extending from including back contact 21 of relay ETR, front contact 22 of relay ETR, upper winding of relay EFS, front contact 55 of relay ED, and back contact 5d of the approach track relay l'WTR, to The picking up of this relay closes stick contact 57 to shunt contacts 55 and 56 out of the circuit just described. Another ick circuit is provided for the relay EFS to maintain that relay picked up as long as the relay ED is maintained picked up. This circuit extends from including the lower winding of relay EFS, front contact 53 of relay ED, and front contact 59 of relay EES, to

With the relays ED and EPS in their picked up positions, a circuit is closed to energize the signal E to cause that signal to be cleared. Thus the Winding 6% of signal E is energized through front contacts 61 and 62 of relays E-FS and El) respectively connected in series.

The shunting of the track section T when an eastbound train accepts the entering signal E prevents further transmission of negative pulses to the opposite end of the block, and thus the relay WI-l at the right-hand end of t e block becomes dropped away. The dropping away of relay WI-l causes cessation of the transmission of positive pulses by the opening of front contact 35 in the circuit for the code transmitter relay WCP. Relay WC? cannot become active at this time for the transmission of negative pulses, if it is assumed that there is no train in approach of signal W, and thus the back contact 64- of the approach relay lETR is open. It is thus provided that upon a train accepting the eastbound signal E, the transmitter at the right-hand end of the block is rendered inactive.

At the left-hand end of the block, the code transmitter relay ECP continues to apply negative pulses to the track rails of the track section T as an independent transmitter, with the rate of transmission determined by the time constant of the shunt circuit for the relay EPL. As is indicated on the drawings, this rate is preferably at a rate of approximately 29 pulses per minute. It will be noted that the rate is determined by the time constant of this shunt circuit because the track relay ETR is inactive at this time and thus the relay EPL is maintained energized during each ti period of the transmitter until the charge of the condenser 51 is substantially dissipated.

Relay ECl continues to be active for the transmission of negative pulses until the rear of the eastbound train has passed out of the approach track section ZWT because the relay AWTR cannot be picked up unless the track relay ZWTR is picked up first to close its front contact 65 in the pick up circuit for relay IWTR. This front contact 65 is shunted upon the picking up of track relay lWTR by the stick contact 66. When relay IWTR becomes picked up under these conditions, the opening of back contact 24 of relay IWTR renders the code transmitter relay ECP inactive, and thus causes the cessation of the transmission of negative pulses at the left-hand end of the track section T. Complete restoration to normal conditions of the code communication apparatus auras-as associated with the block is therefore efiected, even though the block is still occupied by a train.

Assuming further passage of an eastbound train through the block, the entrance of the train into the approach track section ZETR, prior to entering the track seztion IETR, establishes a pick up circuit for the east stick relay ES. This circuit extends from including normally closed contact 67 .of push button jE-RPB, front contact 68 of relay IETR, back contact 69 of relay ZETR and winding of relay ES, to As the train progresses further so as to occupy the approach track section lET, the dropping away of relay EETR establishes a stick circuit to maintain the relay ES picked up until the train-has left the track section lET. This stick circuit includes back contact 68 of relay IETR and stick contact 74} of relay ES.

Relay ES prevents the initiation of coding in the block because of the opening of back contact 71 in the circuit for the code transmitter relay WCP. Thus the code transmitter relay WCP cannot be rendered active for the transmission of negative pulses by the occupancy of the approach track sections lET and ZET by a train leaving the block.

Having thus described in detail the circuit organization that is provided for the control of the signal E for governing eastbound traffic, it is to be understood that a similar organization, and a similar mode of operation is effective for controlling the signal W for westbound trafiic.

It is further provided that cancellation of coding can be rendered effective by actuation of the cancel push button CPB that is provided at each end of the block. Thus, if an eastbound train should enter the track section lWT without intending to pass through the block, as for a switching move, the actuation of the push button W-CPB is eflfective in an obvious manner to cause the picking up of the west stick relay WS to open the circuit for the code transmitter relay ECP at back contact 23 and thus cause cessation of the coding in the block. Relay WS, when picked up under these conditions, is maintained energized, as long as the track section IWT is occupied, by a stick circuit extending from including normally closed contact 72 of push button W-RPB, back contact 73 of relay IWTR, front contact 74 of relay WS and Winding of relay WS, to Thus when the train leaves the track section IWT, relay WS is restored to its normally deenergized position.

Provision is also made for reversing the direction of .trafiic once a train has passed through the block so as to pick up the stick relay WS, for example, at the lefthand end of the block. According to the mode of operation as it has been described, the coding in the block is shut down at this time, and the energization of the stick relay WS prevents the initiation of coding because of occupancy of the approach track sections IWT and ZWT. If, under these conditions, it is desirable :for the train to reverse its direction, coding can be initiated in the block so as to clear the signal E by the actuation of the push button W-RPB so as to open its normally closed contact '72 and cause the dropping away of the stick relay WS. The dropping away of this relay closes back contact 23 in the circuit for the code transmitter relay ECP so as to permit the setting up of coding in the track section T and thus permit the clearing of signal E,

The time locking relays EAS and WAS are provided for preventing a quick reversal in the direction of traffic established through the block. One condition under which the time locking as provided by relay EAS is effective, is a condition where an eastbound train i l proaches signal E and causes the clearing of that signal, but such train does not enter the block because it reverses its direction, as for a switching move. Under these conditions, it is necessary to actuate the cancel button W CPB to cause the signal 13 to be put to stop 7 its actuation to close front contact 75, and thus the relay EAS remains in its dropped away position until the timer ETER has completed its operation.

if a Westbound train should enter the approach track section 1E1 before the relay EAS has been restored to its normally energized position, negative pulses would be transmitted to the left-hand end or the block causing the picking up of the relay EH by the pulsing of contact 76 of the track relay ETR, but the picking up of relay EH would be inefiective to cause the transmission of positive pnlses through the track rails because of the front con,

tact 77 of relay EAS being open. Thus negative pulses are continuously transmitted from the right-hand end of the block, but no positive pulses can be transmitted from the left-hand end of the block for the clearing of the signal W until the timer ETER has completed its operation to permit the restoration of relay EAS to its normally energized position. As soon as relay EAS becomes picked up, the closure of its front contact 77 permits the transmission of positive pulses for the clearing of signal W by a mode of operation corresponding to that which has been described when considering the passage of an eastbound train. 1

Although the signals have been disclosed as providing only two indications, it is to be understood that-a caution indication may be added if required in practice. In order to provide this added indication an additional distinctive code will be required to be selectively transmitted through the block.

Having described the present invention as applied to a typical stretch of railway track as one embodiment of the present invention, it is to be understood that the invention is readily applicable to other track layouts that may be encountered in practice, and that various adaptations, alterations and modifications may be applied to the specific form shown within the spirit or scope of the present invention, except as limited by the apending claims.

What we claim is:

1. In an absolute block signaling system for a stretch of single track constituting a block and having a signal governing entrance to each end of the block, normally inactive track circuit code transmitting and receiving apparatus at each end of the block, an approach track section in approach of each of the signals, means for rendering said code transmitting and receiving. appa atus active to communicate codes in both directions through the track rails of the block; in response to occupancy oi the approach track section at either end of the block for the selective control of the signals, and means for rendering said transmitting means inactive at both ends of the block when a train is within the block and said approach track sections are unoccupied.

2. in an absolute block signaling system for a stretch of single track constituting a block and having signals governing entrance to the block for both directions of traflic, normally inactive track circuit, code transmitting and receiving apparatus connected to the track rails at both ends of the'block, said apparatus being normally effeetive at both ends of the block to maintain the track trails of the block normally deenergized, means for initiating said code transmitting apparatus at either end of the block in response to a train approaching the block at that end, means eiiective upon the reception of track circuit code at th other end of the block for rendering said transmitting apparatus at said other end efiective to transmit code pulses through the block, and means for clearing the signal at the end of the block that is being approached by a train in response to track circuit code received .by said track circuit code receiving apparatus.

3. In an absolute block signaling system for a stretch of single track constituting a block and having signals governing entrance to the block for both directions of trafiic, an approach track section in approach of each of the signals, normally inactive track circuit code transmitting and receiving apparatus connected to the track rails at both ends of the block, said apparatus being normally effective at both ends of the block to maintain the track rails of the block normally deenergized, means for initiating said code transmitting apparatus at either end of the block when a train occupies said approach track section at that end, means effective upon the reception of track circuit code at the other end of the block for rendering said transmitting apparatus at that end active to transmit code pulses through the block, and means for clearing the signal being approached by a train occupying an associated approach track section in response to a track circuit code transmitted through the block.

4. In an absolute block signaling system for a stretch of single track constituting a block and having signals governing entrance to the block for both directions of trafiic, an approach track section in approach of each of the signals, normally inactive track circuit code transmitting and receiving apparatus connected to the track rails at both ends of the block, said apparatus being normally elfective at both ends of the block to maintain the track rails of the block normally deenergized, means for initiating said code transmitting apparatus at one end of the block when a train occupies said approach track section at that end, said means being rendered ineffective when a train occupies the associated approach track section upon leaving said block, means eifective upon the reception of track circuit code at the other end of the block for rendering said transmitting apparatus at that other end of the block active to transmit code pulses through the block, and means for clearing the signal being appreached by a train in response to track circuit code transmitted through the track rails of the block.

5. In an absolute block signaling system for a block comprising a stretch of single track and having signals for governing entrance to the block for both directions of traffic, an approach track section in approach of each of the signals, normally inactive code transmitting and receiving means connected to the track rails at each end of the block, said apparatus being normally effective at both ends of the block to maintain the track rails of the block normally deenergized, said code transmitting means being effective when rendered active to transmit pulses of selected polarity, and said code receiving means being distinctively responsive to the polarity of the pulses received, means for initiating said code transmitting means at either end of the block to transmit pulses of one polarity when a train occupies said approach track section at that end,

means effective upon the reception of track circuit code pulses of said one polarity at either end of the block to transmit pulses of the opposite polarity, and signal clearing means operable in response to the reception by said code receiving means of pulses of said other polarity only.

6. In an absolute block signaling system for a stretch of single track constituting a block and having signals for governing entrance to the block for both directions of traffic, an approach track section in approach of each of the signals, normally inactive track circuit code transmitting and receiving apparatus connected to the track rails at each end of the block, means including one of said approach track sections for rendering said track circuit code transmitting and receiving apparatus active to transmit track circuit code pulses in both directions alternately through the block, signal clearing means responsive to the reception of track circuit code of a particular character for clearing one of said signals, and shut down means operable in; response to a train entering the block to render said code transmitting means inactive at the opposite endof the block, said shut down means being operable in response to a train vacating said approach section after entering the block to render said code transmitting means inactive at the corresponding end of the block.

7. In an absolute block signaling system for a stretch of single track constituting a block and having signals for governing entrance to the block for both directions of traffic, an approach track section in approach of each of the signals, normally inactive track circuit code transmitting and receiving apparatus connected to the track rails at each end of the block, means including one of said approach track sections for rendering said trackcircuit code transmitting and receiving apparatus active to transmit track circuit code pulses in both directions al ternately through the block, signal clearing means responsive to the reception of track circuit code of a particular character for clearing one of said signals, and shut down means operable in response to a train entering the block for rendering said code transmitting means inactive at the opposite end of the block.

References Cited in the file of this patent UNITED STATES PATENTS Re. 22,915 Van Horn Sept. 16, 1947 2,244,901 Staples June 10, 1941 2,274,289 Allen Feb. 24, 1942 2,330,895 Judge Oct. 5, 1943 2,341,300 Thompson Feb. 8, 1944 2,353,421 Staples July 11, 1944 2,365,837 Pelican Dec. 26, 1944 2,488,683 Reichard Nov. 22, 1949 2,605,391 Preston July 29, 1952 2,351,540 Pelican June 13, 1944 

1. IN AN ABSOLUTE BLOCK SIGNALING SYSTEM FOR A STRETCH OF SINGLE TRACK CONSTITUTING A BLOCK AND HAVING A SIGNAL GOVERNING ENTRANCE TO EACH END OF THE BLOCK, NORMALLY INACTIVE TRACK CIRCUIT CODE TRANSMITTING AND RECEIVING APPARATUS AT EACH END OF THE BLOCK, AN APPROACH TRACK SECTION IN APPROACH OF EACH OF THE SIGNALS, MEANS FOR RENDERING SAID CODE TRANSMITTING AND RECEIVING APPARATUS ACTIVE TO COMMUNICATE CODES IN BOTH DIRECTIONS THROUGH THE TRACK RAILS OF THE BLOCK IN RESPONSE TO OCCUPANCY OF THE APPROACH TRACK SECTION AT EITHER END OF THE BLOCK FOR THE SELECTIVE CONTROL OF THE SIGNALS, AND MEANS FOR RENDERING SAID TRANSMITTING MEANS INACTIVE AT BOTH ENDS OF THE BLOCK WHEN A TRAIN IS WITHIN THE BLOCK AND SAID APPROACH TRACK SECTIONS ARE UNOCCUPIED. 